Due to their excellent polymerizability, acrylic monomers can be polymerized by various reactions, such as solution polymerization, suspension polymerization, emulsion polymerization, photopolymerization and UV polymerization.
Many proposals have been made on the production of acrylic polymers; for example, JP-B-55(1980)/401 (the term “JP-B” as used herein means an “examined Japanese patent publication”) discloses a method in which a mixture of an acrylic monomer and a mercaptan is heated at 20 to 200° C. in the presence of oxygen to carry out bulk polymerization; Japanese Patent No. 2582510 discloses a method in which a mixture of an acrylic monomer and a mercaptan that contains substantially no initiator is polymerized in a nitrogen atmosphere; JP-B-2(1990)/55448 discloses a method in which polymerization is conducted at high temperatures (around 150° C.) using an extrusion barrel instead of a batch reactor; JP-A-7(1995)/330815 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a method in which a composition is irradiated with ultraviolet rays using optical fibers in a batch reactor and polymerized by pulse irradiation of ultraviolet rays; and JP-A-11(1999)/49811 discloses a method in which a composition is irradiated with ultraviolet rays in a batch reactor and simultaneously the temperature in the reactor is stepwise raised to carry out UV bulk polymerization.
However, the acrylic monomers have such high reactivity that drastic exotherm occurs in the reactor when those monomers are reacted in an industrially conventional batch reactor with the use of a thermally decomposable polymerization initiator. The heat of reaction is hard to remove from the reaction system so that it has been impossible to conduct bulk polymerization for acrylic monomers under effective control of the reaction when a thermally decomposable polymerization initiator is used.
To carry out the reaction in the barrel, the reaction temperature must be set to relatively higher region. Therefore the accuracy of temperature control decreases, which may cause the result that the resulting polymer has a broad molecular-weight distribution and dispersed molecular weights.
In the case of the reaction in the barrel equipped with an ultraviolet irradiation device, the temperature control is difficult so that the reaction cannot be controlled with high accuracy. Further, it will need considerable expenses to equip the batch reactor having an ultraviolet irradiation device with a heat-release system to control a greater exotherm caused by scaling up of production. As such, this method is not proper for mass production of acrylic polymers utilizing the existing equipment.
The bulk polymerization for acrylic monomers is in a way a preferable form of reaction since the resultant polymer contains no solvent, surface-active agent or the like so that separation of the solvent, etc. from the polymer is not needed and such properties as water resistance are not deteriorated by the surface-active agent or other components.
However, in the bulk polymerization using a thermally decomposable polymerization initiator, it is extremely difficult to control the exothermic polymerization because of the high reactivity of the monomers used. Therefore the polymerization often goes out of control. The runaway reaction is dangerous because the reaction can not be controlled and the condition of the reaction system rapidly changes, as rapid rise of the reaction temperature. Also, the resultant polymers tend to have broad molecular-weight distribution and low molecular weights.
On the bulk polymerization of acrylic monomers, JP-A-53(1978)/258.9 discloses a method for the preparation of thermosetting acrylic resins by polymerizing a mixture of a (meth)acrylate and a crosslinkable monomer, or a syrup thereof. In this method, prepolymerization is carried out in a reaction vessel at temperatures not higher than 150° C. to produce a prepolymer composition having a polymer ratio of at least 60%, and the prepolymer composition is taken out of the vessel and polymerized by multistage polymerization process in which the polymer ratio is increased by 10 to 60% in each stage. In Examples of the above publication, azobisisobutyronitrile and tert-butyl peroxylaurate are each used in an amount of about 0.01 to 0.3 parts by weight based on 100 parts by weight of the monomers. Azobisisobutyronitrile and tert-butyl peroxylaurate have a 10 hours half-life temperature of 66° C. and 98.3° C., respectively. When these thermally decomposable polymerization initiators of high 10 hours half-life temperatures are used at about 0.01 to 0.3 parts by weight based on the monomers, the temperature in the reaction system will rapidly increase as soon as the reaction initiates so that the reaction will run away if a high-performance cooling apparatus is not used. Therefore, this process requires a reactor equipped with a cooling apparatus having a sufficient cooling ability and also requires to carry out the polymerization in multi stages with sufficient cooling in each stage to prevent the runaway reaction. Thus, a high-performance equipment for cooling the reaction system is needed to be provided to carry out the above invention.
JP-A-58(1983)/87171 discloses a process for preparing acrylic pressure-sensitive adhesives having weight-average molecular weights of 100,000 to 600,000. This process comprises two stages in which acrylic monomers are polymerized at 40 to 120° C. with addition of a radical polymerization initiator in an amount of 0.00005 to 0.5 parts by weight based on 100 parts by weight of the acrylic monomers, the radical polymerization initiator having a half-life of 0.1 to 1000 hours at 70° C. and 0.1 to 5 hours at the initial polymerization temperature, respectively (1st stage), and the prepolymer is polymerized at a temperature higher than the 1st-stage temperature and within 100 to 200° C. with addition of a radical polymerization initiator in an amount of 0.0001 to 1 parts by weight that has a half-life of more than 1000 hours at 70° C. and at least 2 hours at the initial polymerization temperature, respectively (2nd stage). The polymerization initiators listed in the above publication include organic peroxides, such as acetyl peroxide, lauroyl peroxide, benzoyl peroxide, diisopropyl peroxide, di-2-ethylhexyl peroxydicarbonate, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxylaurate and tert-butyl peroxyacetate; and azo compounds, such as azobisisobutyronitrile and 2,2′-azobis(2,4-dimethylvaleronitrile).
Although the polymerization initiators used in the above invention are thermal polymerization initiators, 10-hours half-life temperatures thereof are in the range from 43 to 102° C. There is no selectivity on these temperatures for the half-life of 10 hours of the thermal polymerization initiators. Rather, a characteristic aspect in the above invention is the amount of the thermal polymerization initiator, of which the above invention discloses the use in a very small to large quantity. The quantities of heat generated by using the thermal polymerization initiators may differ greatly depending on the activity of the initiator when controlling the reaction temperature within 40 to 120° C. For example, when tert-butyl peroxide or benzoyl peroxide is used as in the Examples, the temperature in the reaction system will rapidly increase as soon as the reaction initiates so that it is requisite for a high-performance cooling unit to control the heat generated in the reaction system.
Japanese Patent No. 2752458 discloses a method for the preparation of methacrylic polymers, which comprises placing a mixture of monomers containing methyl methacrylate in major proportion in a complete-mixing reactor, while adjusting the amount of dissolved oxygen in the monomers to 1 ppm or less, using a radical polymerization initiator with a half-life of 0.5 to 120 seconds at the polymerization temperature, stirring the mixture with a specified stirring force with controlling the mean residence time to make the half-life of the radical polymerization initiator and the mean residence time falling within a specific range.
The radical polymerization initiators used in the Examples of the above publication include 2,2-azobisisobutyronitrile, tert-butyl peroxyisobutyrate and lauroyl peroxide, all of which have a 10 hours half-life temperature over 41° C. Therefore, a high-performance cooling apparatus is used in the Examples of the above invention to prevent the runaway reaction; for example, a heat exchanger cooled by a −5° C. refrigerant is shown in FIG. 2.
As described above, it has been conventional in such bulk polymerization to remove the heat generated from the reaction system by means of a high-performance cooling unit in order to prevent the runaway reaction, with no technical idea on the selection of polymerization initiator. Accordingly, a problem of very expensive cooling unit has been encountered with these methods. Further, in the industrial-scale production of acrylic polymers where reaction hardly proceeds homogenously, it is very difficult to cool the entire reaction tank uniformly even by the use of a considerably high-performance cooling unit. Once the reaction goes out of control in part of the reactor, the runaway reaction could spread all over the reaction system so that such a high-performance cooling unit cannot be always applied to massive industrial production even if it enables stable reaction in the laboratory scale.